How did you move from hourly to flat project rate?

Motivated by the "what's your hourly rate?" poll. There's the famous patio11 post[0] that talks about charging flat rates rather than hourly. I've had some scattered success with this but I'd like to do it a lot more often. There's a wealth of general advice, (esp on Patrick's blog) but contextualizing that with personal stories would be very helpful

For those who have made the transition from hourly to flat rate, how did you get there? For those who tried and found it wasn't effective for you, what do you think the major sticking point was?

[0]https://www.kalzumeus.com/2015/05/01/talking-about-money/